hello,
i have the same problems with IIS, do you find a issue ?
I checked this list :
1.Update upload_tmp_dir and session.save_path in php.ini
2.Create the configured session and upload directories
3.IIS: Assigned permission to create and modify to IIS guest user account
4.Apache: Make sure your webserver has permissions to write into the configured directory
5.Check if the variable "session.use_cookies" in php.ini is set to 1.
6.Check the variable "session.auto_start" in php.ini. If it is set to 1, change it to 0.